The present invention relates to a novel electromagnetic wave absorber, a method of manufacturing the electromagnetic wave absorber, a composite member and an appliance of the electromagnetic wave absorber. More particularly, the invention relates to an electromagnetic wave absorber comprising composite magnetic particles composed of magnetic metal grains and ceramics, particularly, fine crystalline grains containing at least one kind of material selected from the group consisting of non-magnetic or soft magnetic metal oxides, carbides and nitrides. The invention also relates to a method of manufacturing the electromagnetic wave absorber, a composite member using the electromagnetic wave absorber, and a semiconductor device, a printed wire board, an optical sending and receiving module, an electronic toll collection system and an electronic device casing using the magnetic metal particles.
In accordance with the present invention, an optical sending module, an optical receiving module or an optical sending and receiving module integrating the optical sending module and the optical receiving module for use in a high speed communication network using optical fibers can be obtained, and the modules can be made small in size, light in weight, high in processing speed and high in sensitivity by suppressing noises emitted to the outside and noise interference inside the module.
In recent years, there have been significant advances in high speed processing in electronic equipment, and the operating frequency of an IC, such as an LSI or a microprocessor, has rapidly increased, with the result that undesirable noises are likely to be emitted.
Further, in the field of communication, the GHz band electromagnetic waves are proposed for use in the next generation of multimedia mobile communication (2 GHz) and in wireless LANs (2 to 30 GHz). In the field of the Intelligent Transport System (ITS), the Electronic Toll Collection System (ETC) uses 5.8 GHz electromagnetic waves, and the Advanced Cruise-assist Highway System (AHS) uses 76 GHz electromagnetic waves. It is expected that the range of use of high frequency electromagnetic waves will rapidly expand even further in the future.
As the frequency of electromagnetic waves is increased, an electromagnetic wave is apt to be emitted as a noise. Thus, in recent electronic equipment, due to a decrease in the noise margin due to a reduction in the electric power consumed by the equipment and by a decrease in the immunity (noise resistance) due to replacement of digital circuits to analogue circuits and the tendency of small-sizing and high-mounting density, the noise environment inside the equipment has been deteriorated, thereby to cause a problem of erroneous operation of the equipment due to electromagnetic interference (hereinafter, referred to as EMI).
Therefore, measures have been taken to reduce the EMI inside the electronic equipment by placing an electromagnetic wave absorber in the electronic equipment. As an electromagnetic wave absorber for the GHz band, a sheet composed of an electrically insulating organic material, such as rubber, a resin or the like, and a magnetic lossy material, such as a soft magnetic metal oxide, a soft magnetic metallic material or the like, is mainly used.
However, the electric resistivity is around 500 to 1000 μΩ·, which is not so high. Therefore, a decrease of the magnetic permeability due to eddy currents in the GHz region is inevitable. Further, in regard to the complex specific dielectric constant, since the imaginary part is large compared to the real part, because the electric resistivity is not sufficiently high, it is difficult to adjust the impedance matching.
In general, characteristics required for the electromagnetic wave absorber for electronic information-and-communication equipment are {circle around (1)} a large reflection attenuation coefficient (small reflection coefficient), {circle around (2)} a wide band capable of absorbing electromagnetic waves, and {circle around (3)} a small thickness. However, no electromagnetic wave absorber capable of satisfying all of these characteristics has been developed as yet.
In order to attain the above item {circle around (1)}, it is necessary that the amount of electromagnetic waves reflected on the surface of the absorber is small. In order to do so, it is necessary to make the value √{square root over ( )}(μr/∈r) of the characteristic impedance of the substance close to the value √{square root over ( )}(μ0/∈0) of the characteristic impedance of the free space. Therein, μr is a complex specific magnetic permeability μr(μr′+jμr″), ∈r is a complex specific dielectric constant ∈r(∈r′+j∈r″), and μ0 and ∈0 are the magnetic permeability and the dielectric constant of the free space, respectively. In order to attain the above item {circle around (2)}, it is necessary that the values μr′ and μr″ are gradually monotonously decreased with respect to frequency, while the relationship between the values μr′ and μr″ is being kept nearly constant. In order to attain the above item {circle around (3)}, it is necessary that the amount of attenuation of electromagnetic waves inside the substance is made large. In order to do so, it is necessary that the real part of the propagation constant (γ=2πf(μr, ∈r)0.5) of the substance is large, that is, the values of the complex specific magnetic permeability and the complex specific dielectric constant at a desired frequency are made large. However, as the value of the complex specific magnetic permeability becomes large, it is difficult to adjust the impedance matching of the substance with the free space.
Since the soft magnetic metal oxide material of spinel crystal structure as a proven electromagnetic absorber has an electric resistivity extremely higher than that of the soft magnetic metallic material, the magnetic permeability rapidly decreases in the GHz band though the reflection by eddy current is small. Therefore, a considerably large thickness is required in order to well absorb the electromagnetic waves.
On the other hand, the soft magnetic metallic material offers the possibility of providing a thin electromagnetic wave absorber, because the specific magnetic permeability is very high. However, in the high frequency region, the specific magnetic permeability is substantially decreased and the imaginary part of the complex specific dielectric constant is substantially increased due to eddy current loss because the electric resistivity is low. Therefore, the reflection becomes large, and the soft magnetic metallic material does not work as an electromagnetic wave absorber.
In order to solve the problem described above, Japanese Patent Application Laid-Open No.9-181476 proposes to use an ultra-fine crystalline magnetic film of a hetero-granular structure in which ferromagnetic ultra-fine crystalline metallic phases are dispersed in a metal oxide phase as an electromagnetic wave absorber in a high frequency range. Such a magnetic film is characterized in that soft magnetism is provided by the ferromagnetic ultra-fine crystals and high electrical resistivity is provided by the metal oxide phase, and thereby the eddy-current loss is reduced and a high magnetic permeability in the high frequency range can be obtained.
The method of manufacturing the electromagnetic wave absorber is that the soft magnetic metal and oxygen, nitrogen, carbon are sputtered together with a metal oxide phase constitutive element having an affinity with the above elements at a time to form an amorphous film containing these elements on a substrate such as an organic film, and then the film is heat-treated to form a two-phase structure by producing the ferromagnetic ultra-fine crystals in the metal oxide phase. However, the electromagnetic wave absorber has problems in that the cost is high because a large film-forming apparatus is required, and the use of the electromagnetic wave absorber is limited because of the thin-film structure.
Japanese Patent Application Laid-Open No.7-212079 and Japanese Patent Application Laid-Open No.11-354973 disclose an electromagnetic wave interference suppresser or an electromagnetic wave absorber composed of oblate shaped soft magnetic metal particles and organic bond. The soft magnetic metal particles are formed in an oblate shape having a thickness thinner than the skin depth to suppress eddy current, and an improvement in the magnetic resonance frequency is achieved by the effect of shape magnetic anisotropy, and improvement of magnetic permeability is achieved by reducing of the demagnetization field caused by the shape. As the result, an excellent electromagnetic wave absorption ability is obtained in the range of several MHz to 1 GHz. However, it does not have a sufficient thickness and absorption ability as an electromagnetic wave absorber used inside of electronic equipment or used for a high frequency region.
Further, Japanese Patent Application Laid-Open No.9-111421 proposes a magnetic material for loading coils which obtains high electric resistivity in a high frequency region by heat-treating a high magnetic-permeability amorphous alloy at a temperature above the crystallization temperature in an atmosphere containing at least one kind of gas selected from the group consisting of oxygen gas, nitrogen gas and ammonia gas to form crystal grains made of a high magnetic permeability alloy and oxide or nitride around the crystal grains.
Furthermore, Japanese Patent Application Laid-Open No.11-16727 proposes a magnetic thin film for high frequency magnetic elements composed of iron having ferromagnetism and nickel ferrite having magnetism, and having a structure of dispersing a magnetic phase in a ferromagnetic phase or the ferromagnetic phase in the magnetic phase, or laminating the ferromagnetic phase and the magnetic phase in a multilayer arrangement. However, this publication does not propose to use the magnetic thin films as an electromagnetic wave absorber.
Further, Japanese Patent Application Laid-Open No.9-74298 proposes an electromagnetic wave shield material formed by mixing ceramic and magnetic grains in a ball mill using a silicon nitride ball, and then sintering the mixture. However, the publication does not propose any electromagnetic wave absorber.
Further, in regard to the optical sending and receiving module, a preventive measure of internal interference caused by sending and receiving noises between the optical sending part and the receiving part is disclosed in Japanese Patent Application Laid-Open No. 11-196055.